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Memory upgrade compatibility list for iMac 2019-2020

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wilberforce

macrumors 6502
Aug 15, 2020
371
284
Hi guys, read through this topic here and would like to say thanks to all the contributors that made this happen :)
Decided to purchase CT32G4SFD8266 from amazon, 2 sets of (32gbx2) a total of 128gb.

Received my ram today and slotted it in but got some questions I hope you guys can help with. Noted that Lime666 and cczhu pretty similar issue as me. But I am hoping to get some advice on how I should proceed please.

1. I ordered 2 sets of CT32G4SFD8266 (32gb x 2) x 2. I made separate orders for each set. However I received 1 set labeled CT32G4SFD8266 and the other 16ATF4G64HZ. Are these the same? (I checked with crucial chat on the website and they said to contact Amazon as the 16ATF4G64HZ falls under Micron and that the Amazon seller named Crucial is actually a reseller.)

2. When I installed them in the iMac 2020 today, I did 2 configs, AABB one which gave me 2667 speed and the other ABAB which gave me 2133. So was pretty happy when I saw 2667 till I watched a video regarding single and dual channel ram setup which can affect performance? totally lost at this point.

the vid I watched. jump to 6.42 shortcut in the comments

3. What should I do? Have attached a pic in the 2133 setup FYI. Should I go to the AABB setup? which gives me 2666, but if I understand correctly is not dual channel mode.

Hope I didn't miss out some solution/explanation in the previous posts but I read through a few times and where I got confused is in the single channel dual channel thing. where you could still have 2667 speed but diminished performance.

Thanks in advance :)
The only requirement for dual channel is that the sticks in each channel are the same size (GB). (Assuming they have same speed, latency and rank, to avoid problems.) You are fine with AABB. This has been tested and confirmed by several people in these forums, including by me.
You can convince yourself by running Novabench RAM speed test several times (with no other processes running). You should get about 28000 MB/s if running in dual channel at 2667 MHz.
 
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robbie1056

macrumors newbie
Nov 11, 2020
10
2
hi all.i am new to all this and just ordered 27 inch 2020 imac,never used mac ios either before
i want to upgrade the ram and wondered if any of the below is better or not
just to confirm if someone can just say yes or no.
1, i cannot add ram in 2 slots and keep the existing apple 2 x 4gb in the other 2 slots
2. 2 x 16gb in 2 slots only and 2 empty(if so what slots?)
3. 4 x 8gb in all 4 slots
even though option 2 and 3 would be the same 32gb would either make a difference
i was going to use crucial-do i need to go for the option with the M at the end?i.e

Crucial CT2K8G4S266M 16 GB Kit (8 GB x2)​

as it will be a new imac-am i right in installing the ram before i switch it all on
thanks for any advice
i have been reading the posts but a little confused
 
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wilberforce

macrumors 6502
Aug 15, 2020
371
284
hi all.i am new to all this and just ordered 27 inch 2020 imac,never used mac ios either before
i want to upgrade the ram and wondered if any of the below is better or not
just to confirm if someone can just say yes or no.
1, i cannot add ram in 2 slots and keep the existing apple 2 x 4gb in the other 2 slots
2. 2 x 16gb in 2 slots only and 2 empty(if so what slots?)
3. 4 x 8gb in all 4 slots
even though option 2 and 3 would be the same 32gb would either make a difference
i was going to use crucial-do i need to go for the option with the M at the end?i.e

Crucial CT2K8G4S266M 16 GB Kit (8 GB x2)​

as it will be a new imac-am i right in installing the ram before i switch it all on
thanks for any advice
i have been reading the posts but a little confused
Some answers:
1. Technically, you can add RAM in 2 slots and keep the 2x4GB in the other two slots, but memory speed will be degraded. Thus most prefer to remove the Apple RAM, as it is better to run the RAM at full speed, rather than have slightly more RAM all running at a lower speed. Short answer: you are correct.
2. Put 2x16GB in slots 2 and 4 (where slot 1 is at top).
3. 4x 8GB in all four slots will also work, but there is no advantage. (Some claim this is slightly faster, but that is very debatable, as the normal variation in test results is more than any perceptible difference). It will cost slightly more than 2x16GB, and also leaves you with no open slots if you want to add more RAM in the future.
4. The crucial kit that you cite is a good choice, and the M suffix indicates it is "for Mac" and guaranteed as such. However, many have used the non-M version with no problems, and surmise it is just marketing/environmental, as there does not appear to be any real substantive difference (maybe more testing). However, instead of 4x8GB, I would suggest this 2x16GB kit which is what i am using : CT2K16G4S266M

5. Many other kits work just as well. I tested this one and it was just as good, and is a great price, and is also guaranteed for Mac:

6. I actually suggest do not install the RAM before you turn it all on. Make sure it all works fine with the Apple RAM first. Then once everything is installed, running to your satisfaction, turn it off and switch out the RAM. Important note: after changing RAM, the Mac will take extra time to boot up, and appear to be completely black and unresponsive for an extra minute: do not worry, it is not broken! It is also quite easy to not get the RAM seated properly.
Hope this helps.
 
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robbie1056

macrumors newbie
Nov 11, 2020
10
2
Hi Wilberforce
thanks very much for the in depth reply,really useful and just what i needed
also the advice on starting up once i take the mac out the box and run everything first then install the ram
think ill go for the crucial 2x16 gb option
im only gonna use it to surf the web,emails looks at some photos etc,,not going to really edit anything,would 32gb be too much too start
thank you
 
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wilberforce

macrumors 6502
Aug 15, 2020
371
284
Hi Wilberforce
thanks very much for the in depth reply,really useful and just what i needed
also the advice on starting up once i take the mac out the box and run everything first then install the ram
think ill go for the crucial 2x16 gb option
im only gonna use it to surf the web,emails looks at some photos etc,,not going to really edit anything,would 32gb be too much too start
thank you
Nowadays 2x16GB is a kind of sweet spot for price.

But for your use case, 2x8GB would probably be fine.
Unless you are one of those that like to have lots (like dozens) of tabs open in a web browser, which can can consume remarkably large amounts of RAM - Chrome is notorious for this.

You might consider staying with the stock 2x4GB for a while, and keep an eye on the "memory pressure" in the Activity Monitor. If it gets out of the green zone, or it shows high "Swap Used," you need more RAM. There is nothing wrong with using only 8GB for simple tasks, I have been using 8GB on my Macbook for this purpose for 5 years no problem.

Activity-Monitor-Memory-Pressure.png
 
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Megabass

macrumors newbie
Nov 5, 2020
8
1
Moscow, Russia
Some answers:
1. Technically, you can add RAM in 2 slots and keep the 2x4GB in the other two slots, but memory speed will be degraded.
The problem of low speed is solved by installing memory modules in the same channel as Apple memory.
Example:
1 slots 4Gb Apple
2 slots 4GB Apple
3 slots New RAM
4 slots New RAM
In this case, the speed will be maximum
 
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wilberforce

macrumors 6502
Aug 15, 2020
371
284
The problem of low speed is solved by installing memory modules in the same channel as Apple memory.
Example:
1 slots 4Gb Apple
2 slots 4GB Apple
3 slots New RAM
4 slots New RAM
In this case, the speed will be maximum
For the benefit of others, this arrangement is not entirely correct:
Apple RAM comes installed in Slots 2 and 4, not 1 and 2.
If the new RAM is not the same size as Apple RAM, dual channel mode is lost (it uses flex mode instead).
 
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Megabass

macrumors newbie
Nov 5, 2020
8
1
Moscow, Russia
For the benefit of others, this arrangement is not entirely correct:
Apple RAM comes installed in Slots 2 and 4, not 1 and 2.
If the new RAM is not the same size as Apple RAM, dual channel mode is lost (it uses flex mode instead).
It does not matter what channels Apple RAM stands for - the main thing is that it is installed with new modules. There are many examples on youtube, here is one of them
The user has 4 RAM modules: 2x4 Apple and 2x8 Crucial, in total 24GB at the frequency 2666Mhz
 
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wilberforce

macrumors 6502
Aug 15, 2020
371
284
It does not matter what channels Apple RAM stands for - the main thing is that it is installed with new modules. There are many examples on youtube, here is one of them
The user has 4 RAM modules: 2x4 Apple and 2x8 Crucial, in total 24GB at the frequency 2666Mhz
See timestep 7:03 in this video for explanation why you should not mix RAM sizes in the 2020 iMac

It is not a problem for other iMacs, only for the 2020 iMac
 
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robbie1056

macrumors newbie
Nov 11, 2020
10
2
For the benefit of others, this arrangement is not entirely correct:
Apple RAM comes installed in Slots 2 and 4, not 1 and 2.
If the new RAM is not the same size as Apple RAM, dual channel mode is lost (it uses flex mode instead).
hi wilberforce-if i kept the 2 x apple ram and added another 2 4gb ram in the other two slots would this work,or does it have to be all the same size and brand..thanks
 
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wilberforce

macrumors 6502
Aug 15, 2020
371
284
hi wilberforce-if i kept the 2 x apple ram and added another 2 4gb ram in the other two slots would this work,or does it have to be all the same size and brand..thanks
Yes that will work. If it is a different brand use order AABB, as long as all are same size.
 
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wilberforce

macrumors 6502
Aug 15, 2020
371
284
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kungfuthug

macrumors newbie
Nov 12, 2020
5
0
any one ordered from crucial.com in the past? how long does it take for them to ship? My order has been "in process" for more than 24 hours. Ordered my CT2K32G4S266M from there cause Amazon didn't have it in stock.
Hi. How did that RAM work in your 2020 iMac? I have the ability to buy 64GB for cheap. Does it work?
 
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x83

macrumors newbie
Nov 10, 2020
3
2
The only requirement for dual channel is that the sticks in each channel are the same size (GB). (Assuming they have same speed, latency and rank, to avoid problems.) You are fine with AABB. This has been tested and confirmed by several people in these forums, including by me.
You can convince yourself by running Novabench RAM speed test several times (with no other processes running). You should get about 28000 MB/s if running in dual channel at 2667 MHz.

Hi Wilberforce, thank you for your reply! Switched to AABB and ran nova benchmark as you recommended.

Thanks for highlighting the requirement that all the ram just needs to be the same size.

Based on what you mentioned, looks like everything is sweet. Have attached the pic for anyone who might find the info useful, included the highest and lowest score.
 

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x83

macrumors newbie
Nov 10, 2020
3
2
You got ram from a 3rd party seller. Always, ALWAYS buy only from Amazon.com. It should say "Sold by and shipped by Amazon.com" on the right side. Anything else is a gamble, like you just found out. This "reseller" (who is pretending to be crucial) shipped you whatever was on the shelf, hoping you wouldn't notice.

Send it all back and buy a new set, ensuring you are actually buying from Amazon. Don't forget to leave a one-star review on the seller page because these jokers need to be flushed out.
Hi Warp9,

thank you for the advice regarding resellers, got mislead by thinking that there could be only one crucial username on amazon. My mistake.

Def gonna leave that review as you recommended. Just wanted to say thanks!
 
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